SpamAssassin Pro. "for MS Outlook 2000/XP is built on the most popular and successful anti-spam filtering software available: SpamAssassin." ($19.95) [evhead]

I find this a little annoying. Why is it that free software on unix has to become pay software for the PC world? Not only that, this is an open source project that anyone can work on. Does anyone who work on it get a cut of money that is made? How does this work exactly?